Photolithography tools are used to transfer a pattern on a mask onto a wafer or a glass substrate. During this course, the wafer or the glass substrate is carried by a wafer stage to move horizontally in synchronization with a mask stage, with its vertical position being focused and leveled in accordance with settings of a focusing and leveling system (FLS). Errors in movement of the wafer stage can pose a direct impact on imaging quality of a projection objective. In order to reduce vertical focusing and leveling errors, the vertical position of the wafer stage is maintained by a vertical motion module which is supported by low-rigidity gravity compensators for blocking the transmission of low-frequency vibrations from a base frame to the vertical motion module. A typical design of such a low-rigidity gravity compensator is as disclosed in the patent document CN201010571517.8. Patent document CN201010618373.7 describes a typical wafer stage.
As disclosed in the patent document CN201010618373.7, precise vertical positioning of the wafer stage is realized by controlling it based on grating scales, i.e., on relative-position feedbacks. Wherein, the establishment of a vertical zero (initial) position of the wafer stage is effectuated by a zeroing process during the initialization. In addition, prior to the zeroing process, an initial vertical position of the wafer stage, i.e., its physical position, also known as an absolute position, is ensured by the gravity compensators. However, as the gravity compensators are controlled using a pressure control loop without any feedback loop for the absolute position, the initial position is significantly susceptible to pressure fluctuations in the gas supply and disturbance introduced by a load, and the lower the rigidity of the gravity compensators, the greater deviations in the initial vertical relative position will result from the gas-supply pressure fluctuations and load-caused disturbance.
Possible consequences of a considerable drift in the initial vertical position include: 1) failure of the wafer stage's initialization upon pneumatic vibration of a cylindrical gas bearing in the wafer stage; 2) considerable vertical zeroing errors during the initialization, which may lead to significant zero position deviations of the wafer stage relative to the whole system; and 3) an inferior vertical motion precision after the initialization.
Test data showed that a vertical deviation of the above-described wafer stage was up to 0.3 mm, exceeding the maximum permitted value that was 0.1 mm. Although this can be mitigated by increasing the accuracy of control valves, it is impossible to eliminate its root causes in this way. Currently, the primary cause of insufficient reliability of a wafer stage lies in deviations in its initial vertical position which are attributable to the absence of absolute zero-position references.